In recent years, LTE (Long Term Evolution) system has been proposed as a cellular system. The LTE system uses SC-FDMA (Single Carrier-Frequency Division Multiple Access) system as a wireless access system in the uplink direction, and OFDMA (Orthogonal Frequency-Division Multiple Access) system as a wireless access system in the downlink direction.
The OFDMA system is a digital modulation/demodulation system in which radio resources are divided into a plurality of frequency bands using the orthogonality of frequencies, data are multiplexed with carriers having divided frequency bands (referred to as subcarriers), and the resultant signals are transmitted. The OFDMA system is alleged to have tolerance against fading and multi-path interference.
In contrast, although the SC-FDMA system is similar to the OFDMA system, the former is different from the latter in the following points. In the OFDMA system, divided frequency bands are allocated to individual users, whereas in the SC-FDMA system the radio resources are divided into frequency bands and time components and the divided radio resources are allocated to individual users. Moreover, in the SC-FDMA system, the frequency bands allocated to the users are longer than those in the OFDMA system. Thus, the SC-FDMA system is expected to improve power efficiency in comparison with the OFDMA system.
In addition, a wireless base station in the LTE system transmits a pilot signal that includes cell identification information that identifies a cell that its base station manages. A mobile terminal receives the pilot signal so as to identify a cell in which its terminal is present and performs a handover. The handover means that a mobile terminal switches from one base station to another base station through which the mobile terminal communicates.
FIG. 1 is a sequence diagram describing the operation of a cellular system in the state that a handover is performed. Hereinafter, a wireless base station that manages a cell in which a mobile terminal has been moved is referred to as a moving source wireless base station, whereas a wireless base station that manages a cell to which the mobile terminal has been moved is referred to as a moving target wireless base station.
Step 0: The moving source wireless base station stores context information of mobile terminals present in the cell of its base station (UE context). The context information includes roaming restriction information that is information with respect to wireless base stations that mobile terminals cannot access. The roaming restriction information is provided from a mobile terminal to the moving source wireless base station when an RRC connection is established between the mobile terminal and the moving source wireless base station or a timing advance is updated. The timing advance is information with which transmission timing of an uplink signal transmitted from a mobile terminal is controlled.
Step 1: The moving source wireless base station selects cells to measure the received signal strength (RSSI: Received Signal Strength Indication) using the context information stored at step 0. The moving source wireless base station transmits measurement control information (Measurement Control), that causes the received signal strengths of the selected cells to be measured, to the mobile terminal.
Step 2: When the mobile terminal receives the measurement control information, the mobile terminal measures the received signal strength of each of the selected cells. Specifically, the mobile terminal receives pilot signals from wireless base stations that manage the selected cells and measures the received signal strength of each of the cells based on the pilot signals. When the received signal strength becomes equal to or greater than a predetermined threshold, the mobile terminal transmits to the moving source wireless base station a measurement report that denotes that the received signal strength indication becomes equal to or greater than threshold target.
Step 3: When the moving source wireless base station receives the measurement report, the moving source wireless base station starts performing a handover process.
Step 4: The moving source wireless base station transmits to the moving target wireless base station a handover request message that causes a handover to be performed.
The handover request message includes information with respect to X2 interface, information with respect to S1 interface, information with respect to SAE (System Architecture Evolution) bearer, and information that represents settings of RRC. The X2 interface is an interface that mutually connects wireless base stations, whereas the S1 interface is an interface that mutually connects a wireless base station and an EPC (Enhanced Packet Core). On the other hand, the information with respect to the SAE bearer includes Qos information.
The moving target wireless base station extracts a transport layer address of the moving source wireless base station from the information with respect to the X2 interface. In addition, the moving target wireless base station extracts a transport layer address of the EPC from the information with respect to the S1 interface.
Step 5: The moving target wireless base station performs an admission control that admits a mobile terminal using the Qos information included in the handover request message. In addition, the moving target wireless base station reserves a C-RNTI (Cell Radio Network Temporary Identifier) and a dedicated random access preamble and decides a validation period for the dedicated random access preamble. Moreover, the moving target wireless base station generates a transparent container.
Step 6: The moving target wireless base station transmits an acknowledge reply, that acknowledges the handover request message (Handover Request Acknowledge), to the moving source wireless base station. The transparent container transmitted to the mobile terminal is included in the acknowledge reply.
Step 7: When the moving source wireless base station receives the acknowledge reply, the moving source wireless base station transmits a handover command, that causes the handover to be performed (RRC Handover Command), to the mobile terminal. The handover command includes the transparent container transmitted from the moving target wireless base station.
Step 8: When the mobile terminal receives the handover command, the mobile terminal synchronizes with the moving target wireless base station on the basis of each radio frame. When the mobile terminal has been assigned a dedicated random access preamble, the mobile terminal executes a non-contention based random access procedure. In contrast, when the mobile terminal has not assigned a dedicated random access preamble (for example, when the moving target wireless base station has used all dedicated random access preambles), the mobile terminal executes a contention based random access procedure.
Step 9: The moving target wireless base station transmits token frame Ta that prevents mobile terminals from interfering on UL (Up Link)-SCH (Synchronization Channel) by a RACH (Random Access Channel). In addition, the moving target wireless base station informs the mobile terminal of the allocated UL-SCH resources.
Step 10: When the mobile terminal receives the allocated UL-SCH resources, the mobile terminal transmits confirmation information that confirms a handover (Handover Confirm) to the moving target wireless base station using the UL-SCH resources. The moving target wireless base station collates C-RNTI included in the confirmation information and C-RNTI transmitted to the moving source wireless base station at step 6. Thus, the moving target wireless base station confirms that a handover of the mobile terminal, to which the resources have been allocated, has been completed. At this point, the moving target wireless base station starts transmitting data to the mobile terminal.
Step 11: The moving target wireless base station transmits a path switch message to an MME (Mobility Management Entity) to inform it that the mobile terminal has moved to another cell.
Step 12: When the MME receives the path switch message, the MME transmits a user plane update request message to an S-GW (Server Gateway).
Step 13: When the S-GW receives the user plane update request, the S-GW switches the path of the down link to the moving target wireless base station. Thereafter, the S-GW releases the U-Plane/TNL resources with respect to the moving source wireless base station.
Step 14: The S-GW transmits a reply to the user plane update request (User Plane Update Response) message to the MME.
Step 15: When the MME receives the reply message, the MME transmits a path switch acknowledge reply (Path Switch Ack) message to the moving target wireless base station.
Step 16: When the moving target wireless base station receives the path switch acknowledge reply message, the moving target wireless base station transmits a release request that causes resources to be released (Release Resource) to the moving source wireless base station to inform the moving source wireless base station that a handover has been successfully performed.
Step 17: When the moving source wireless base station receives the release request, the moving source wireless base station releases the resources. Now, the handover has been completed.
At present, the LTE system is continuously transmitting pilot signals. The continuous transmission of the pilot signals does not mean that the pilot signals are perfectly continuously transmitted, but means that the pilot signals are transmitted at very large transmission repetitions. Hereinafter, the continuous transmission is referred to as a regular transmission.
When the pilot signals are regularly transmitted, since the transmission power of the pilot signals increases, the power consumption of the wireless base stations also increases. This leads to a large cost factor in the operation of the cellular system. Thus, it has been desired to accomplish a technology that suppresses the transmission power of the pilot signals and that suppresses the power consumption of the wireless base stations.
Patent Document 1 describes a pilot channel transmission method that can suppress the transmission power of pilot signals. In this pilot channel transmission method, the pilot signals are intermittently transmitted. The repetition rates at which pilot signals are intermittently transmitted are lower than those at which they are regularly transmitted. Hereinafter, transmission that is intermittently performed is referred to as the intermittent transmission.
FIG. 2 is a descriptive diagram showing the transmission power of a pilot signal and the power consumption of a wireless base station in each regular transmission state and intermittent transmission state.
Transmission power 1 represents the chronological change of the transmission power of a pilot signal in the continuous transmission state, whereas transmission power 2 represents the chronological change of the transmission power of a pilot signal in the intermittent transmission state. On the other hand, power consumption 3 represents the chronological change of the power consumption of a wireless base station in the continuous transmission state, whereas power consumption 4 represents the chronological change of the power consumption of the wireless base station in the intermittent transmission state.
Transmission power 1 is always a large value since the pilot signal is continuously transmitted. Transmission power 2 is a large value for a transmission period in which the pilot signal is transmitted since the pilot signal is intermittently transmitted, however, transmission power 2 becomes zero at the transmission intervals of the pilot signal. Thus, the integral value of transmission power 2 is smaller than that of transmission power 1. Thus, the integral value of power consumption 4 is smaller than that of power consumption 3. Note that since the wireless base station does not stop its operation at the transmission intervals of the pilot signal, power consumption 4 does not become zero even at the transmission intervals.